An engine generally includes a transmission device, which transmits power from the crankshaft to one or more camshafts by means of a chain, a belt or the like, as shown in FIG. 15. Similar power transmission devices are also used in other machines, especially where a driven shaft is to be rotated by a driving shaft which is parallel to, but spaced some distance from, the driven shaft. In the transmission device of FIG. 15, a movable guide Ga, in sliding contact with a circulating power transmission medium, which in this case is an endless CH, cooperates with a tensioner T to maintain appropriate tension in the chain, and also prevents vibration in the plane of travel of the chain as well as transverse vibrations.
The movable guide Ga is attached to a frame E of the engine on a supporting shaft P, which may be a mounting bolt, a pin or the like. In FIG. 15, a driving sprocket S1, which may be a crankshaft sprocket, operates driven sprockets S2, which may be camshaft sprockets, through the chain CH. A fixed guide Gb is also provided for limiting and guiding the travel of the circulating chain CH.
FIG. 10 is a side elevational view a plastic movable guide 300, used as a tensioner lever in a chain transmission, which the present inventor previously described in Japanese Patent Application No. 2000-382798, and FIG. 11 is a bottom plan view of the plastic movable guide.
In this plastic movable guide 300, a guide body 301 includes a shoe 302, with the surface of which a traveling chain CH is sliding contact. The guide body also includes a plate-receiving portion 303 extending along the back of the shoe 302 along the longitudinal direction of the guide. The shoe and plate-receiving portion are integrally molded as a unit from a synthetic resin. A plate 308, composed of a rigid material, typically metal, reinforces the guide body 301, fits into a slot 307 in the plate receiving portion. This slot extends along the longitudinal direction of the guide and has its opening facing in a direction opposite to the direction in which the chain-engaging surface of the shoe faces. Amounting hole 305, for mounting the guide on the frame of an engine or other machine, is provided in the plate-receiving portion near one end of the guide. A through hole 308A is provided in the reinforcing plate 308 near one end thereof. These holes, 305 and 308A, are in register with each other when the reinforcing plate is inserted into the slot 307 in the plate-receiving portion, and both receive a supporting shaft P (FIG. 15).
Since the guide body 301 itself incorporates a shoe on which the chain slides, it is not necessary to provide a separate shoe member. Thus, the integrally molded construction reduces the number of parts required for the guide, and also reduces the number of required production steps. The reinforcing plate 308 increases the strength, toughness and bending rigidity of the guide in the pivoting direction.
In the plastic movable guide 300, the relationships between the respective sizes and shapes of the guide body 301 and the reinforcing plate 308 are not particularly considered, and the mounting hole 305 was designed so that its diameter was the same as that of the through hole 308A. However, variations in accuracy in production, and the differences between coefficients of thermal expansion of materials, either caused the reinforcing plate 308 to protrude slightly from the slot, or to retract into the slot by causing the reinforcing plate 308 to become slightly smaller than the guide body 301. Various phenomena occur as a result of these variations.
When the above-mentioned plastic movable guide was attached to the frame of an engine and used as a tensioner lever, a higher level of noise occurred compared the level of noise occurring where a movable guide composed entirely of plastic was used. Accordingly, an improvement in quietness is desired.
There was also a danger that the integrally molded, synthetic resin guide-body would become worn and deteriorate, finally reaching a stage at which it would fracture when struck by the plunger of the tensioner. In order to reduce the rate of wear and deterioration, it was believed necessary to incorporate fibers in the synthetic resin, or to subject the synthetic resin to a cross-linking process or the like, both of which measures resulted in increased production cost.
The inventor has studied and analyzed the causes of noise and deterioration in plastic movable guides, and has found that the above-described problems are not generated in the same manner in all plastic movable guides. When the reinforcing plate is retracted into the plate-receiving portion of the guide body at the location of the plunger-receiving portion, as shown in FIG. 12(a), the reinforcing plate 308 moves back and forth in the slot 307 of the guide body 301 every time the plunger 310 hits the guide body 301. As a result the end portion of the reinforcing plate is brought into contact with the front surface of the plunger as shown in FIG. 12(b) and the level of metallic noise that occurs is higher than in the usual case, where the reinforcing plate is in flush relationship with the opening of the slot. Further, the inventor also found that the other edge of the reinforcing plate 308 hit the bottom of the slot, i.e., the back surface of the shoe, as shown in FIG. 12(a), as the plate moved back and forth in the slot 307. The repeated engagement of the edge of the plate 308 with the back of the shoe resulted in cracks in the synthetic resin and caused deterioration of the shoe.
As shown in FIG. 10, the profile of the plunger-receiving portion 301B of the plastic movable guide 300 is a straight line. Thus, the contact angle between the front end of the plunger and the plunger-receiving portion changes in use as the plunger projects and the guide rotates about its pivot axis. As shown in FIGS. 13(a) and 13(b), the contact area between the front end of the plunger and the plunger-receiving portion decreases, and load is concentrated at one portion of the plunger-receiving portion. The inventor has found that the concentration of the load was a cause of accelerated wear in the plunger-receiving portion of the guide. Additionally it has become clear that, while using the plastic movable guide, vibration can cause the movable guide, or the plunger, to become inclined relative to the plane of rotation of the plunger about its pivot axis. When this inclination occurs, as shown in FIGS. 14(a) and 14(b), load is concentrated at a corner of the edge of the reinforcing plate, causing acceleration of wear of the reinforcing plate.
Accordingly, the objects of this invention are to solve the above-mentioned problems encountered in the use of prior plastic movable guides, and to provide a plastic movable guide having superior quietness of operation, reduced production cost, and comparatively little wear of the reinforcing plate over a long period of time.